1. Field of the Invention
This invention relates to generally to the field of three-dimensional imaging. More specifically, the invention relates to an intra-operative method of visualizing boli during a bone cement injection procedure.
2. Background of the Invention
Percutaneous bone cement injection procedures such as vertebroplasty are currently some of the least invasive treatments available for bone surgery. For example, in vertebroplasty, an operator simply injects bone cement i.e. poly(methyl methacrylate) (PMMA), using a syringe passing through the skin, through the pedicles and into the anterior vertebral body. The bone cement forms a bolus which hardens to become much stiffer than bone, thus raising the strength of the vertebra. Success of the procedure is typically determined by restoration of vertebral height and removal of pain, which are typically unrelated to the two major types of complications, cement leakage and over-reinforcement of bone. While fluoroscopic x-ray imaging, commonly used intra-operatively, can aid in the avoidance of leakage, in order to avoid over-reinforcement the procedure needs to be optimized such that less cement is required. Currently, exploration of parameters such as bolus location and shape, important steps in optimization, are only explored in the research setting and will remain academic matters until they can easily be cost-effectively observed and controlled in the clinical setting.
Cement volume is the single most important factor in determining the strength and stiffness increase in fractured bone. Generally, the more cement injected, the stronger the reinforcement, but the higher risk of complications. Aside from leakage, it has been found that many patients suffer fractures in adjacent vertebrae at a later date. This side effect may be a result of the higher stiffness of the reinforced vertebra compared to the surrounding vertebrae. The difference in stiffness may cause high stresses in the immediately superior and inferior bones which are often already weakened by a systemic condition such as with osteoporosis. Thus, the amount injected is typically limited, and alternative, less stiff materials have been explored. To further reduce the amount of fill necessary to reinforce a vertebral body, one needs to take into consideration the geometry of the bolus; if stronger reinforcement can be achieved by alternative arrangements of the cement, less total material is necessary. In order to implement such measures, appropriate visualization tools are necessarily, because the operator must make adjustments during the procedure as the specific features of the treated bone influence where the cement flows during injection.
Computed tomography (CT), occasionally performed before the procedure, yields an accurate three-dimensional picture of the bone, and if post-operative, of the bone cement within the bone. However, CT scanners are somewhat expensive to use, take additional time to schedule and perform, and are not always immediately available. Fluoroscopic CT, while an ideal technology for the procedure being real-time and capable of watching for stray channels of cement and calculating the location of the bolus, is quite expensive and less available than CT. Because of the sheer number of vertebroplasty procedures preformed, on-site monitoring of the procedure is thus typically implemented via X-ray fluoroscopy, with which the operator can gain a real-time, two dimensional picture of the current condition inside the bone. This helps to visualize needle position and depth and can identify certain problems such as cement leakage, but does not offer much insight into the three-dimensional positioning and shape of the bolus. Additionally, the two dimensional picture does not provide any information regarding the optimal volume of the bolus.
Consequently, there is a need for a simple, inexpensive, intra-operative method to monitor the 3-D position and shape of a bone cement bolus during a percutaneous bone cement injection procedure.